This report has been prepared by Explicit ApS on behalf of the Danish Environmental Protection Agency to assess current NOX emission levels from ships in Danish waters and explore possible models for enforcement of the NECA using the same at-sea surveillance techniques currently used for sulphur emissions monitoring. The NOX data used in this report has been collected over a four-year period and consist of 2,249 ship observations in which ship exhaust plumes have been sampled using sniffer techniques deployed via airborne surveillance. Due to its magnitude, the dataset allows for a unique insight into the real sailing NOX emissions – including differences in patterns between Tiers, engine and ship types – not seen before. In addition to the gas sample data, external data has also been sourced on various engine parameters to understand impacts from speed, engine size, and fuel efficiency on the NOX emissions. The RSE data is thus presented as a load-adjusted NOX emission factor (g/kWh) for each ship observation. A brief introduction to NOX formation in ship engines and possible abatement techniques is also presented.